Add USE_CLANG_LLD and use_clang_lld.

* USE_CLANG_LLD is unedefined in current builds.
* When USE_CLANG_LLD is defined to 'true' or '1',
  use clang's lld instead of ld or ld.gold.
* When lld is enabled:
  * ld-only flags are not passed to 'lld'.
  * location_packer is disabled.
  * Use new lld's --pack-dyn-relocs=android.
* When lld does not work:
  * In Android.mk files use LOCAL_USE_CLANG_LLD := false.
  * In Android.bp files use use_clang_lld: false.
* Only arm, arm64, x86, and x86_64_devices have LLD flags;
  all other hosts and targets do not call lld yet.

Bug: 73768157
Test: make checkbuild and boot
Change-Id: I06b8a1e868a600997a7e70fe05c299d751d23d5f
diff --git a/cc/config/toolchain.go b/cc/config/toolchain.go
index 7961575..ca863a7 100644
--- a/cc/config/toolchain.go
+++ b/cc/config/toolchain.go
@@ -66,6 +66,7 @@
 	ClangCflags() string
 	ClangCppflags() string
 	ClangLdflags() string
+	ClangLldflags() string
 	ClangInstructionSetFlags(string) (string, error)
 
 	ndkTriple() string